Compared with metallic glasses, polymeric glasses have superior glass forming ability, low glass transition temperature (Tg) and wider supercooled liquid region (ΔT, defined as the difference between Tg and the onset crystallization temperature Tx), thus have a very wide range of applicability. The thermoplasticity nature of common glassy polymers is exploited in molding and imprinting. Since chemical scientists invented the thermoplastics in 1940's, they became the basic materials for the 2nd industrial revolution. Although their strength is only about one fiftieth of that of steels, thermoplastics products became very cheap because they could be prepared repeatedly using the same mold at a temperature near room temperature. Now thermoplastics are widely used in our daily lives.
In the early 1960's, non-crystalline alloys (so called metallic glasses) were firstly fabricated in laboratories. Compared with polymers, metallic glasses also have advantages in mechanical, electrical, magnetic and chemical properties. Amorphous metal alloys have a supercooled liquid region ΔT above Tg. When an amorphous metal alloy was heated into this region, it still keeps its glassy state and does not crystallized immediately. In general, the wider range of ΔT suggests better deformability of the supercooled state. Therefore, the stability against crystallization is crucial for deformation of an amorphous metal alloy in its supercoolied state. The stability during a heating process is closely related with the critical cooling rate necessary for forming a glass from its liquid. For a good glass former, it is expected that the time-temperature-transformation (TTT) curve will move toward the long time direction.
The conventional melt-spin glasses have a limited ΔT and could not be used to investigate the related properties of the supercooled liquid region. In the early 1990's, bulk metallic glasses (BMGs) with large size up to millimeters even centimeter scales in three dimensions were developed using conventional casting methods. For most of BMGs, their values of ΔT are larger than 45 K and even larger than 100 K in some case. The supercooled liquids of these BMGs show typical Newtonian flow characteristics at low strain rates or low stresses and the maximum elongation could reach about 15000%. For crystalline alloys, they are unable to deform as easily as the viscous supercooled liquids of BMGs, which are capable of remaining the glassy structure and original properties even after large plastic deformation. The unique combination of these superior properties and homogeneous microstructure makes BMGs a new type of engineering materials in applications such as manufacturing the micro-electro-mechanical components. Meanwhile, high strain rates and superplasticity are suitable for quality control, thus make it possible for the mass production.
For most of the known BMGs, however, their industrial applications are still impeded by the limitation of alloy size and the lack of workability and machinability. The polymerlike exploitation of the supercooled viscous flow of many BMGs was also postponed due to low Tg and low stability against crystallization. Additionally, some BMGs based on noble metals like Pd, Pt and Au can only be used in the experimental laboratory because of their high cost although they have good glass forming ability and superplasticity in the supercooled liquid region.